1. Field of the Invention
The present invention relates generally to heart-stimulation devices, and particularly to R-wave inhibited, P-wave rate controlled devices.
2. Description of the Prior Art
In the field of medical electronics, heart pacers are becoming widely accepted and well known at this juncture. There are several varieties of heart pacer available on the market today. There are pacers which provide stimulation to the ventricle of the heart at a fixed rate, pacers which provide stimulation to the ventricle of the heart on demand, pacers which provide stimulation to the atrium and ventricle of the heart in a predetermined timing sequence and on demand, and pacers which provide stimulation to the ventricle in response to signal inputs received from the patient's body other than from the ventricular chambers of the patient's heart. (By no means is this listing intended to include all types of pacers available today.) It is with this last mentioned type of heart pacer that the present invention is more particularly related.
In some prior art, signal inputs from the thorax, such as an electrical signal analogous to breathing rate, is described to control the stimulation rate of the device. In other prior art even more closely related to the present invention, sensing of the P-wave is used to control the time when ventricle stimulation can occur. For instance a device is known which can detect P-waves and which causes a ventricular pulse generator to supply a stimulation pulse at a fixed interval of time after the P-wave detection occurred. If no P-wave occurs, the pacer provides a stimulation pulse to the ventricle of the heart at the end of a predetermined interval from the previous ventricular heart beat or stimulation. If a P-wave is sensed at or near the time of occurrence of the ventricular stimulation impulse, the ventricular stimulation impulse still occurs at the expected time. There is thus an upper limit on the interval between the ventricular stimulation impulses, or in other words, a lower limit to rate.
But consider the effect of the P-wave occurring early in the escape interval. The ventricular stimulation impulse is thus made to occur at a fixed interval after the detection of the P-wave and is thus earlier than otherwise it would have occurred. Accordingly, such prior art represents only a shortening of the escape interval (or in other words, a speed-up of the stimulation rate), but does not provide a slowing down of the rate when it would be desirable to do so, for example when a patient tries to relax or sleep. If the pacer is provided with a basic rate sufficiently slow to accommodate relaxation or sleep and may "speed-up" therefrom in response to P-wave rate increase, there remains the problem of P-wave generation and/or detection results in reversion to a basic rate which is too slow to be acceptable and may require surgical replacement.
Another pacer which operates in a somewhat similar manner is also known in the prior art. There too, however, the ventricular stimulation pulses occur at a certain basic rate in a "free-running" mode and the atrial pulses and accompanying circuitry operate only to accelerate the ventricular stimulation pulse rate (i.e. decrease the escape interval).